Retaining Wall Calculator: Blocks, Gravel, Fabric and Cost by Length

1. Enter the wall length and wall height in feet.
2. Choose or adjust the block size in inches. Standard landscape blocks are 18 x 6 inches.
3. Set a waste factor (10 percent is typical for straight walls, 15 percent for curves).
4. Adjust the drain zone width if your design differs from the standard 12 inches.
5. Optionally enter unit costs to get a total material cost estimate.
6. Read the full materials list: blocks, caps, gravel, fabric, drain pipe and adhesive.

A retaining wall calculator takes the length and height of a planned segmental block wall and works out every material you need: wall blocks, cap blocks, drainage gravel, geotextile fabric, perforated drain pipe and cap adhesive. It replaces guesswork with exact geometry so you can order the right quantities and get an accurate cost estimate before you start digging.

Segmental retaining walls use interlocking concrete blocks that stack without mortar. They are the most common DIY retaining wall type for walls up to about 4 feet high. Taller walls usually require geogrid reinforcement and engineered design, which are beyond the scope of this calculator.

For related material estimates see the construction calculators hub, the gravel calculator or the concrete calculator.

The calculator uses straightforward geometry to convert your wall dimensions and block size into a complete materials list.

1. Convert the wall length from feet to inches and divide by the block face length to find blocks per course, rounding up.
2. Convert the wall height to inches and divide by the block face height for the number of courses, rounding up.
3. Multiply courses by blocks per course to get the raw block count, then add the waste percentage and round up.
4. Cap blocks equal one course worth of blocks across the top of the wall.
5. Gravel volume is the drainage zone behind the wall: length times height times drain width, converted to cubic yards.
6. Fabric covers the back of the wall plus 2 feet of overlap at the top and bottom.

Why Rounding Up Matters

You cannot buy a fraction of a block. Rounding every count up to the next whole number means you always have enough material, even when the wall length is not an exact multiple of the block size.

Take a wall that is 20 feet long and 3 feet high, using standard 18 x 6 inch landscape blocks with 10 percent waste and a 12-inch drainage zone.

Blocks per course: ceil(20 x 12 / 18) = ceil(13.33) = 14 blocks.

Courses: ceil(3 x 12 / 6) = ceil(6) = 6 courses.

Raw blocks: 14 x 6 = 84 blocks.

With 10% waste: ceil(84 x 1.10) = ceil(92.4) = 93 blocks.

Cap blocks: 14 (one full course on top).

Gravel: 20 x 3 x (12/12) / 27 = 60 / 27 = 2.222 cubic yards.

Drainage fabric: 20 x (3 + 2) = 100 square feet.

Drain pipe: 20 + 2 = 22 feet.

Adhesive: ceil(20 / 5) = 4 tubes.

If wall blocks cost $3.50 each, caps $5.00, gravel $45/cu yd, fabric $0.50/sq ft, pipe $2.00/ft and adhesive $6.00/tube, the total is about $593.49.

Segmental retaining wall blocks come in several standard sizes. The calculator defaults to the most common landscape block but you can enter any custom dimensions.

Block depth does not affect the face area calculation but it does affect how much soil the wall can retain and whether geogrid is needed. Always check the manufacturer specs for your chosen block.

Proper drainage is the single most important factor in retaining wall longevity. Water pressure behind a wall is the leading cause of failure.

Gravel Backfill

A zone of clean, crushed gravel behind the wall lets water drain down to the base instead of building up hydrostatic pressure. The standard width is 12 inches, but wider zones are better for clay soils or tall walls.

Landscape Fabric

Geotextile fabric lines the gravel zone to keep soil from migrating into the gravel and clogging it over time. The calculator adds 2 feet of overlap so the fabric extends above and below the gravel zone.

Perforated Drain Pipe

A perforated pipe sits at the base of the gravel zone and carries collected water away from the wall. The extra 2 feet allows the pipe to daylight past the end of the wall.

Material costs vary widely by region, supplier and block style. The calculator uses your own unit prices so the estimate matches your local market. Here are typical price ranges as a starting point.

These ranges are typical retail prices in the US as of 2025, provided as a reference. Enter your actual supplier quotes in the calculator for an accurate total. Labor, base gravel, leveling sand and delivery fees are additional.

Check Local Building Codes

Many jurisdictions require a permit for retaining walls over 4 feet high. Some areas set the limit at 3 feet. Check before you build.

Prepare the Base

A compacted gravel base trench 6 inches deep and 24 inches wide is standard. The first course of blocks sits partly below grade for stability. This calculator covers the wall face and drainage, not the base trench.

Plan for Setback

Most segmental blocks have a built-in setback, meaning each course steps back slightly from the one below. This batter adds structural stability. The calculator counts full-size blocks; the setback does not change the block count.

Account for Curves

Curved walls require more cuts and therefore more waste. Increase the waste percentage to 15 or 20 percent for walls with tight curves.

Consider Geogrid

Walls over 3 to 4 feet usually need geogrid reinforcement layers between courses. Consult the block manufacturer guidelines or a structural engineer for taller walls.

Several variables change how many blocks and how much drainage material a retaining wall requires.

Wall Height

Taller walls need more courses of blocks and proportionally more gravel backfill. A wall twice as high uses roughly twice the materials and may also require geogrid reinforcement.

Block Size

Larger blocks cover more face area per unit, so you need fewer of them. An 18 x 8 block uses 25 percent fewer courses than an 18 x 6 block for the same wall height.

Waste Percentage

Straight walls waste about 10 percent of blocks to cuts and breakage. Curved or stepped walls can waste 15 to 20 percent. Choosing the right waste factor avoids a second material run.

Drain Zone Width

A wider gravel zone improves drainage but increases the volume of gravel needed. Clay soils or wet sites benefit from an 18-inch or wider zone instead of the standard 12 inches.

Soil Conditions

Heavy clay retains more water and puts more pressure on the wall. Sandy or well-drained soils are easier on the structure and may allow a narrower drain zone.

Planning a DIY Garden Wall

Before you visit the home improvement store, enter your wall dimensions to know exactly how many blocks and bags of gravel to load on the cart.

Getting Contractor Quotes

An independent materials estimate lets you compare contractor quotes on equal footing and spot any significant overages or underestimates.

Budgeting a Landscaping Project

Enter your local unit costs to see the total material spend before you commit. This helps you decide between a short wall and a taller design.

Ordering Delivery

Blocks are heavy and gravel is sold by the cubic yard. Accurate quantities prevent both shortages and expensive leftover material sitting in your driveway.

1. Skipping Drainage

The most common retaining wall failure is caused by water buildup behind the wall. Always install gravel backfill, fabric and a drain pipe.

2. No Compacted Base

Stacking blocks on bare soil leads to settling and leaning. A level, compacted gravel base is essential.

3. Underestimating Waste

Running out of blocks mid-project means a second delivery trip. Use 10 percent waste for straight walls and 15 to 20 percent for curves.

4. Building Too Tall Without Engineering

Walls over 4 feet need geogrid and possibly an engineered design. DIY walls built too high without reinforcement can fail suddenly.

5. Forgetting Cap Adhesive

The top course must be glued down so it does not shift when people sit on or walk along the wall. One tube per 5 linear feet is the standard rule.

The block count and material calculations are exact geometry. The limitations come from real-world variations in block size, site conditions and construction technique.

Divide the wall length in inches by the block length to get blocks per course, divide the wall height in inches by the block height for the number of courses, then multiply. Add 10 percent for waste on straight walls.

Multiply the wall length in feet by 12 to convert to inches, then divide by the face length of one block and round up to the next whole number.

Cap blocks equal one full course across the top of the wall. If your wall is 20 feet long with 18-inch blocks, you need ceil(240/18) = 14 cap blocks.

Multiply wall length by wall height by the drain zone width (in feet), then divide by 27 to convert to cubic yards. A 20 ft x 3 ft wall with a 12-inch drain zone needs about 2.2 cubic yards.

Landscape or geotextile fabric wraps around the gravel backfill to prevent soil from clogging the drainage zone. The calculator adds 2 feet of overlap above and below the gravel.

Use 10 percent for straight walls and 15 to 20 percent for curved walls or complex layouts where more blocks will be cut.

Yes. A perforated pipe at the base of the gravel zone carries water away from the wall. Without it, water pressure can push the wall forward over time.

Material costs depend on block style, gravel prices and your region. Enter your own unit costs in the calculator for an accurate estimate. A typical 20-foot, 3-foot wall might cost $400 to $800 in materials alone.

Most segmental block walls can be built without engineering up to 3 or 4 feet. Taller walls typically require geogrid reinforcement and may need a permit and professional design.

Standard 18 x 6 inch landscape blocks work for most garden and terrace walls. Use 18 x 8 blocks to reach the same height with fewer courses, or 12 x 4 small blocks for low decorative borders.

• NCMA Segmental Retaining Wall Design Manual (National Concrete Masonry Association; design and construction standards for segmental retaining walls).
• ICPI Segmental Retaining Walls Best Practices Guide (Interlocking Concrete Pavement Institute; installation and drainage best practices).
• Allan Block Installation Guide (Manufacturer installation manual covering base preparation, drainage and block layout for segmental walls).